Q: What is the purpose of this document?

A: We acknowledge that there is an enormous interest in the largest optical-infrared telescope ever conceived, and that ESO’s scientific, technical and industrial community is extensive. ESO is committed to providing as detailed information about the Extremely Large Telescope (ELT) as possible.

Q: Is the ELT fully funded?

A: The funds committed at the December 2014 ESO Council Meeting cover the construction costs for a fully working 39-metre aperture telescope with a suite of instruments that will be the most powerful in the world.

The procurement of some components was moved to a second phase of the project, which would require additional funding, for instance when another Member State joins ESO.

In December 2017 ESO’s governing body, the Council, authorised additional spending to cover the cost of both the five inner rings of segments for the main mirror (M1) of the Extremely Large Telescope (ELT), and one spare set of 133 mirror segments (one-sixth of the total M1), and also an additional mirror segment maintenance unit.

Q: What is the date of ELT first light?

A: First light for the ELT remains targeted for 2025. All major contracts placed as of mid-2018 (more than 30) are compatible with this deadline. This is however very challenging and there is a natural risk that difficulties along the way impact this target. This is unavoidable when building such a one-of-a-kind huge and complex machine.

Q: What are the first big ELT contracts?

A: Most of the major ELT contracts have already been placed. These include:

Construction of the road and platform to ICAFAL (Chile) in late-2013 in anticipation of Council green light

Construction of the M4 Unit to ADOPTICA (Italy) in mid-2015

During 2015, ESO signed agreements with consortia around Europe for the construction of three first-light ELT instruments as well as an adaptive optics system.

In July 2015 ESO signed a contract with the French optics company Reosc, a subsidiary of Sagem, Safran group to manufacture the deformable shell mirrors that will comprise the telescope’s fourth mirror (M4).

A contract for the construction of the telescope’s dome and main structure was awarded in May 2016 to the ACe consortium of Astraldi and Cimolai. This was the largest contract in ESO’s history.

In July 2016 a contract with Safran-Reosc was placed for the polishing of the telescope’s difficult convex secondary mirror (M2).

In January 2017, ESO signed four contracts — with SCHOTT, SENER Group, and the FAMES consortium (Fogale and Micro-Epsilon) — for major components of the ELT. These included the casting of the telescope’s giant secondary and tertiary mirrors, the cells to support these mirrors, and the supply of edge sensors that are vitally important for the functioning of the ELT’s enormous 39-metre primary mirror.

In May 2017 contracts for the manufacture of the 39-metre primary mirror of ESO’s Extremely Large Telescope (ELT) were signed. The German company SCHOTT will produce the blanks of the mirror segments, and the French company Safran Reosc will polish, mount and test the segments.

Q: What are the benefits for Chile of hosting the largest telescope in the world?

A: ESO has a long tradition of close and very fruitful collaboration with Chile — its government, universities, science centres and industry. The main benefit for the Chilean astronomical community is observing time on the world’s biggest eye on the sky. Besides this, a Chilean university has already participated in an international team that developed one of the concepts for the first generation of ELT instruments. It is reasonable to expect that the emerging Chilean scientific community will take similar initiatives in the next few years in related areas, such as engineering or the development of instrumentation.

Q: What are the main differences between the ELT and the existing telescopes?

A: The Extremely Large Telescope (ELT) will have a 39-metre mirror (almost half the length of a football pitch) and will thus be by far the biggest telescope in the world to observe in the visible and the near-infrared (there are larger radio telescopes). The current largest optical telescopes have diameters of up to ten metres, and the ELT's diameter will thus be four times greater. This diameter was chosen because it is the minimum diameter needed to achieve some of the driving science cases. For example, the ELT will be able to image rocky exoplanets and to characterise their atmospheres, while the VLT can only indirectly detect such Earth-like planets. Moreover, the ELT will be able to measure the acceleration of the expansion of the Universe directly. Adaptive optics systems are fully incorporated into the design of the telescope to compensate for the fuzziness in the stellar images introduced by atmospheric turbulence. The ELT will have more than 5000 actuators that can change the shape of its mirrors a thousand times per second.

Q: Which site has been selected for the ELT?

A: The ESO Council selected Cerro Armazones as the site for the Extremely Large Telescope. Armazones is a peak in the Chilean Atacama Desert, with an altitude slightly above 3000 metres. It is located roughly 20 km away from Cerro Paranal, home of the Very Large Telescope, and is another exceptional site for astronomical observations. In anticipation of the choice of Cerro Armazones as the future site of the ELT and to facilitate and support the project, the Chilean Government has agreed to donate to ESO a substantial tract of land contiguous to ESO’s Paranal property and containing Armazones in order to ensure the continued protection of the site against all adverse influences, in particular light pollution and mining activities.

Q: What was the process leading up to the decision for the ELT site and who took the final decision?

A: The independent ELT Site Selection Advisory Committee (SSAC) has been analysing in great detail results from several possible sites worldwide. Similar efforts have been carried out by the Thirty-Meter Telescope (TMT) site selection team from the US. For the sake of efficiency, the sites pre-selected by the TMT team (all in North and South America) were not studied by the SSAC, as the TMT team shared their data with the SSAC. The SSAC drew up a short list comprising four sites in Chile and one in the Canary Islands, Spain. Two of the sites on the SSAC shortlist, including Armazones, were on the TMT list.

On 2–3 March 2010, the ELT Site Selection Advisory Committee presented their report to the ESO Council, confirming that all the sites examined in the final shortlist have very good conditions for astronomical observing, each one with its particular strengths. The technical report concluded that Cerro Armazones, near Paranal, stands out as the clearly preferred site, because it has the best balance of sky quality across all the aspects considered, and it can be operated in an integrated fashion with the existing ESO Paranal Observatory. The ESO Council, ESO’s governing body, convened on 26 April 2010, and, taking into account the recommendations of the Site Selection Advisory Committee and all other relevant aspects, selected Cerro Armazones.

Q: When did construction of the ELT start?

A: Construction of the road and platform started in early 2014. The M2 and M3 mirror blanks were cast in 2017 and the first segments of the main mirror were cast in 2018. The construction work on Cerro Armazones is also progressing well.

Q: What is the cost of the ELT?

Q: What is the operating cost of the ELT?

A: The ELT will be operated as an integral part of the ESO observatories. The operating cost includes not only the cost of running the observatory in Chile, but also the cost of operation support in Garching as well as re-investment costs for telescope upgrades and new instruments/cameras for the telescope. The total operating cost is estimated to be 50 million euros per year.

Q: For how long will the ELT be used?

A: The current operation plan foresees that the ELT will be used for at least 30 years. This is a typical lifetime for such a large facility and implies, as is the case for example at the Very Large Telescope, regular maintenance and a programme of new instrument development. Note that ESO’s La Silla Observatory will have its 50th anniversary in 2019 and is still in operation and highly productive.

Q: Where will the ELT money be spent?

A: The money will be primarily spent in the ESO Member States through industrial contracts. A fair industrial return to all partners is an important component of the endeavour and is carefully evaluated at all phases of the project.

Q: Why spend such a considerable amount of money on astronomical research?

A: Astronomy contributes to our cultural and economic well-being in a number of ways. It is an integral part of our culture and contributes to a better understanding of our fragile environment. Astronomers tackle key questions that challenge our minds and our imagination. How did the planets form? Is life ubiquitous in the Universe? What is the Universe made of? What are dark matter and dark energy?

Beyond these questions, astronomy often inspires young people to choose natural sciences as a career, from where they go on to scientific and technical careers in academia and industry in a wide range of other fields, and thus contributes to a balanced, future-oriented society.

Astronomy is also a modern, high-tech science relying on a strong collaboration with industry to realise challenging large-scale engineering tasks. This brings benefits to both sides.

The ELT, as an example, is a high technology science-driven project that incorporates many innovative developments, offering numerous possibilities for technology spin-off and transfer, together with challenging technology contract opportunities and providing a dramatic showcase for European industry. It will create many high technology jobs.

Q: Why do we need such a large telescope as the ELT?

A: The current generation of large telescopes has allowed astronomers to make tremendous discoveries, opening up whole new areas of study. For example, the current generation of 8–10-metre class telescopes allowed us to take the first pictures of planets orbiting around other stars. Our knowledge in astronomy continues to progress at an incredible pace, answering many questions, but also raising exciting new ones. The ELT will address these new questions, but will also make discoveries that we cannot even imagine yet.

Q: Why is the ELT a 39-metre telescope?

A: The size of a telescope is important for two reasons: one is the amount of light it can collect and the other is the level of detail it can see. As a 39-metre telescope, the ELT will gather 15 times more light than the largest optical telescopes operating today. It will also provide images 15 times sharper than those from the Hubble Space Telescope. The ELT performance is thus orders of magnitude better than the currently existing facilities. Such a telescope may, eventually, revolutionise our perception of the Universe, much as Galileo's telescope did, 400 years ago.

This diameter turned out to be the minimum diameter needed to achieve some of the driving science cases: to image rocky exoplanets to characterise their atmospheres and to directly measure the acceleration of the expansion of the Universe.

Q: Aren’t earthquakes a problem for the ELT?

A: The ELT Project Office has considered the seismic risk in all of the design aspects of the telescope. The quantification of seismic risk was the subject of extensive analysis at the time of the selection of Paranal for the VLT. The design criteria for the ELT follow closely those used for the VLT in terms of accelerations, but take into account the most recent international norms for their application. Two studies were commissioned to re-evaluate the design criteria and this work was reviewed by three independent teams of experts. In addition to these studies, four independent contractors developed options for seismically isolating the telescope and the dome. Most of the resulting precautions against rapid accelerations for the telescope, its optical systems and the dome, would have to be in place in any case to avoid accidental damage. These protections do not carry large budgetary impacts for the telescopes.

Q: How much light will the ELT be able to gather?

A: The ELT alone will gather more light than all of the existing 8–10-metre class telescopes on the planet combined, and 100 million times more light than the human eye, and will be able to detect objects millions of millions of times fainter.

Q: What are researchers hoping to find and achieve with the ELT?

A: The ELT will tackle the biggest scientific challenges of our time, and aim for a number of notable firsts, including tracking down Earth-like planets around other stars in the "habitable zones" where life could exist — one of the Holy Grails of modern observational astronomy. It will also perform ”stellar archaeology” in nearby galaxies, as well as make fundamental contributions to cosmology by measuring the properties of the first stars and galaxies and probing the nature of dark matter and dark energy. On top of this astronomers are also planning for the unexpected — new and unforeseeable questions will surely arise from the new discoveries made with the ELT. The ELT may, eventually, revolutionise our perception of the Universe, much as Galileo’s telescope did, 400 years ago.

Q: What are the main differences between the ELT and existing telescopes?

A: The Extremely Large Telescope (ELT) will have a 39-metre mirror (almost half the length of a soccer pitch) and will thus be by far the biggest telescope in the world to observe in the visible and the near-infrared (there are of course larger radio telescopes). In other words, it will be the world’s biggest “eye” on the sky. The current largest optical telescopes have a diameter of about 10 metres, and the ELT will thus be four times greater. This diameter was chosen because it is the minimum diameter needed to achieve some of the driving science cases: to image rocky exoplanets to characterise their atmospheres, and to measure the acceleration of the expansion of the Universe directly. The main principle behind the telescope is that it is an adaptive telescope. Adaptive mirrors are incorporated into the optics of the telescope to compensate for the fuzziness in the stellar images introduced by atmospheric turbulence. One of these mirrors is supported by more than 5000 actuators that can distort its shape a thousand times per second.

Q: How will the ELT and other facilities work together?

A: Given the geographical proximity between Cerro Paranal — home of the VLT/VLTI and VISTA — and the site chosen for the ELT, Cerro Armazones — they are 20 km apart — the future ELT observatory will be operated in an integrated fashion with ESO’s Paranal Observatory. Adding the transformational scientific capabilities of the ELT to the already tremendously powerful integrated VLT observatory guarantees the long-term future of Paranal as the most advanced optical/infrared observatory in the world and further strengthens ESO’s position as the world-leading organisation for ground-based astronomy. The synergy between the ELT and Paranal will thus be both operational and scientific. Other scientific synergies have been identified and will be crucial, in particular with ALMA and the James Webb Space Telescope (JWST), but also with the Square Kilometre Array and the Large Synoptic Survey Telescope. The synergy between the ELT and the JWST will be very similar to the very successful synergy between the Hubble Space Telescope and ESO’s Very Large Telescope. Astronomers have known for many years that to comprehend the Universe, they need to combine observations done at various wavelengths and make observations both in space and on the ground.

Q: When and how did ESO decide to build the ELT?

A: In December 2004, the ESO Council defined ESO’s highest priority strategic goal as “the retention of European astronomical leadership and excellence in the era of Extremely Large Telescopes (ELT)”, asking that “the construction of an ELT on a competitive timescale be addressed by radical strategic planning”. Following an extensive international review in October 2005 of a first concept study — the OWL project — the ESO project offices conducted a new study in 2006, produced with the help of more than 100 astronomers, to carefully evaluate performance, cost, schedule and risk. In November 2006, the results were subject to detailed discussions by more than 250 European astronomers at a conference in Marseille. Their enthusiastic welcome paved the way for the decision by the ESO Council to move to the crucial next phase of detailed design of the full facility.

Q: Why jump directly from four 8-metre telescopes (VLT) to a much bigger 39-metre ELT?

A: The current generation of large telescopes has allowed astronomers to make tremendous discoveries, opening up whole new areas of study. For example, the current generation of 8–10-metre class telescopes allowed us to take the first pictures of planets orbiting around other stars. Our knowledge in astronomy continues to progress at an incredible pace, answering many questions, but also raising exciting new ones. To address these new questions, but to also make discoveries that we cannot even imagine yet, it is necessary to significantly increase the sensitivity and the angular resolution of our facilities. This is why astronomers worldwide have shown the need for extremely large telescopes in the 30- to 60-metre range. The 39-metre mirror turned out to be the minimum diameter needed to achieve some of the driving science cases: to image rocky exoplanets, to characterise their atmospheres, and to directly measure the acceleration of the expansion of the Universe. ESO has thus decided to work towards consolidating a design for a 39-metre telescope.

Q: Will the ELT be able to take direct images of “mature” exoplanets around stars like the Sun?

A: Discovering and characterising planets and protoplanetary systems around other stars will be one of the most important and exciting aspects of the ELT science programme. This will include not only the discovery of planets down to Earth-like masses using the radial velocity technique, but also the direct imaging of larger planets and possibly even the characterisation of their atmospheres. The ELT will be capable of detecting reflected light from mature giant planets (Jupiter to Neptune-like) and may be able to probe their atmospheres through low-resolution spectroscopy. It will also enable us to directly study planetary systems during their formation from protoplanetary discs around many nearby very young stars. Furthermore, observations of giant planets in young stellar clusters and star-forming regions will trace their evolution as a function of age. Thus, the ELT will answer fundamental questions regarding planet formation and evolution, the planetary environment of other stars, and the uniqueness (or otherwise) of the Solar System and the Earth.

Q: Will astronomers use the ELT to study Solar System bodies, like Mars, Neptune, Pluto or Kuiper-Belt Objects?

A: As the ELT will have a much larger sensitivity and resolution than the current generation of large telescopes, it will certainly be most useful in the study of faint objects in the Solar System. The VLT has already made many discoveries concerning Neptune, Pluto and Kuiper-Belt Objects, and there is no doubt that the ELT will play a very important role there as well.

Q: Will the ELT be able to see the first galaxies that ever formed?

A: The ELT will pursue a vigorous scientific programme of exploring the formation and evolution of galaxies in the distant Universe. With the enormous sensitivity and resolution gains of the ELT we will be able to peer beyond our present horizons and uncover the physical processes that form and transform galaxies across cosmic time. The ELT will provide us with spatially resolved spectroscopic surveys of hundreds of massive galaxies all the way out to the most distant galaxies presently known, supplying us with the kind of detailed information on their stellar masses, ages, composition, star formation rates and dynamical states that is currently only available for nearby galaxies. The ELT will also unveil the crucial earliest stages of galaxy formation, right at the end of the “Dark Ages” after the Big Bang, by identifying the galaxies responsible for the re-ionisation of the Universe and by informing us of their basic properties. Through these observations, the ELT will drive the transition from the current phenomenological models to a much more physical understanding of galaxy formation and evolution.

Q: What about the OWL 100-metre project?

A: The Overwhelmingly Large Telescope (OWL) was a concept study, in the same way as carmakers are making concept cars. It has helped the ELT project teams to explore new ways to build telescopes, and many of these technologies are now considered for the ELT. ESO has no current plan to build a 100-metre optical telescope.

Q: What kind of advantages will the ELT offer, compared with the larger virtual telescope VLTI?

A: By combining four telescopes using interferometry, the achieved resolution is the same as a single telescope whose diameter is equal to the maximum distance between the individual telescopes in the interferometer. In this way, the VLTI allows us to create a virtual telescope up to 130 metres in diameter when combining the 8.2-metre Unit Telescopes (UTs), or up to 200 metres in diameter when combining the 1.8-metre Auxiliary Telescopes (ATs). In spite of this, the 39-metre ELT will have considerable advantages with respect to the VLTI. First, the ELT will have a collecting area that is a hundred times larger than the four VLT ATs combined. This means that the ELT will collect one hundred times more light and therefore will allow observations of much fainter objects. Secondly, in order to create images from a virtual telescope, the VLTI has to observe over a long period of time. Thinking of the four 1.8-metre ATs as a tiny fraction of the full 200-metre virtual telescope, the gaps in the 200-metre virtual mirror must be "filled in" by moving the ATs between positions on the mountaintop, or allowing the Earth's rotation to reorient them relative to the astronomical target. For the ELT the "aperture" is fully filled at all times. Last, but not least, the data from the VLTI must go through a very complex form of data processing. This method only provides a very tiny field of view and currently only works well on relatively bright objects. In conclusion, the VLTI is very powerful for certain applications, but can only compete with the ELT on a fraction of science projects.

Q: Could the ELT take a picture of the Moon-landing sites?

A: The short answer is yes, the sites could be imaged, but no, the images would not be detailed enough to show the equipment left behind by the astronauts. The longer reply needs to consider several technical aspects.

Firstly, the resolution of the 39-metre ELT, using its built-in adaptive optics and observing at visible wavelengths, will correspond to about 10 metres on the surface of the Moon (5 milliarcseconds). However, the parts of the lunar modules that are left on the Moon are less than 10 metres in size. So, the ELT is not big enough to show the shape of the lander, even if it could be imaged, and it would be indistinguishable from a rock. A perfect telescope of about 200 metres in diameter would be needed to show the landers as anything more than an unresolved blob and resolve their shapes.

As an aside — the VLT, when used as an interferometer (VLTI), reaches the required equivalent size of 200 metres, and hence the needed resolution, but it cannot be used to observe the Moon.

Secondly, the contrast between the landers and the brilliant lunar surface is low in almost all circumstances, which makes the imaging problem even harder.

Thirdly, to achieve the finest resolution (image sharpness) the ELT needs adaptive optics. The ELT adaptive optics system relies on natural or artificial guide stars, which would not work in the case of trying to image the lunar surface as the surface is very bright.

The shadow cast by the landers when the Sun is low in the sky would be much longer than 10 metres, which might make the problem a little easier, but they are still narrow. The landers also catch the Sun briefly when the surrounding plain is still in darkness, at sunrise or sunset. In these cases, they would appear as a point of light that might be seen — but could still not be distinguished from a boulder or other natural light-coloured surface feature of the Moon.

You may be wondering whether the NASA/ESA Hubble Space Telescope would have better luck. In fact, while space telescopes are not affected by the atmosphere of the Earth, they are not substantially closer to the Moon. Also, Hubble is much smaller than the ELT, so isn’t able to obtain images that show the surface of the Moon with higher resolution. Read this FAQ for more information. The sharpest recent images of the lunar landers have been taken by the Lunar Reconnaissance Orbiter: Apollo Landing Sites Revisited, from lunar orbit!